Fuse Blowing on Step-down Transformer

[Errorsfordays]

This circuit consists of a pad mount transformer stepping down 34.5kV to 12kV, a 12kV switch-gear, and a secondary transformer stepping down 12kV to 400V and 690V. One fuse keeps blowing on a single phase of the 690V side every time a moderate load is put on it. I have watched the voltage on all three phases while the fuse blows, and there are no voltage drops out of tolerance. Some of the more sensitive components on that line are also burning up intermittently. I have tested every component on that line, and megged the transformer. My question is: Is it possible for a loose connection in the pad mount or switch-gear upstream from the secondary transformer to trip that fuse? With the clock ticking on this fault, I don't want to go down that rabbit hole if its not possible.

 

[jim dungar]

Fuses blow based on downstream/load issues, not upstream ones.

Have you checked the wiring and loads? The drop in voltage could be indicative of a short circuit.

 

[TwoBlocked]

Is it the same "moderate load" that is blowing the fuse each time?

But I am not answering YOUR question. Is it possible for a loose connection in the pad mount or switch-gear upstream from the secondary transformer to trip that fuse? If that is what's happening, then some sparking on the primary side would be causing increased current on the load side, like an inrush to a reactive component. Since "Some of the more sensitive components on that line are also burning up intermittently." yeah, I could see that as being possible.

A thorough thermal scan might show something.

 

[TX+ MASTER#4544]

This circuit consists of a pad mount transformer stepping down 34.5kV to 12kV, a 12kV switch-gear, and a secondary transformer stepping down 12kV to 400V and 690V. One fuse keeps blowing on a single phase of the 690V side every time a moderate load is put on it. I have watched the voltage on all three phases while the fuse blows, and there are no voltage drops out of tolerance. Some of the more sensitive components on that line are also burning up intermittently. I have tested every component on that line, and megged the transformer. My question is: Is it possible for a loose connection in the pad mount or switch-gear upstream from the secondary transformer to trip that fuse? With the clock ticking on this fault, I don't want to go down that rabbit hole if its not possible.

You may have a serious grounding and bonding issue at the transformer(s).
Which will cause a neutral imbalance on any 120 volt single phase volt loads. Sounds like the voltage isn't stabilized, which is what the neutral does for the system. And it severally affects sensitive electronic equipment.
Also, consider installing an SPD at the transformer.
Article 242 Part III Surge Arresters, Over 1,000 Volts. Surge arresters are very similar to SPDs.
Check for bonding jumpers and your grounding electrode conductors at the transformers.

Thanks,
TX+MASTER#4544

 

[petersonra]

It could be just about anything at this point. You will just have to start hunting down the possible causes and eliminate them one by one.

My first pass would be a thorough visual inspection of everything. Tug on all the wires looking for loose or broken ones. Look carefully at every termination for evidence of shorts, arcing, or hot insulation.

Could be something external too like a capacitor bank switching.

And don't let yourself be fooled by what you or someone else thinks is happening.

 

[Errorsfordays]

You may have a serious grounding and bonding issue at the transformer(s).
Which will cause a neutral imbalance on any 120 volt single phase volt loads. Sounds like the voltage isn't stabilized, which is what the neutral does for the system. And it severally affects sensitive electronic equipment.
Also, consider installing an SPD at the transformer.
Article 242 Part III Surge Arresters, Over 1,000 Volts. Surge arresters are very similar to SPDs.
Check for bonding jumpers and your grounding electrode conductors at the transformers.

Thanks,
TX+MASTER#4544

Thank you, this is very helpful. There are fuse blocks and SPDs on both the 690V and 400V sides. At the time of the original failure, the fuse block exploded and continued to arc for at least three seconds. It has all been replaced since then, but I’m not sure if the integrity of the cables to the block has been checked.

 

[electrofelon]

You may have a serious grounding and bonding issue at the transformer(s).
Which will cause a neutral imbalance on any 120 volt single phase volt loads. Sounds like the voltage isn't stabilized, which is what the neutral does for the system.

I'm not really following this. Sounds like a wye connected 690Y/400 secondary. The wye connections are all permanent and factory made inside the transformer. Even if the SBJ is missing, the neutral to phase and phase to phase voltages will all be the same and stable. Even if the system is truly floating, then of course neutral o ground could drift from 0 a bit, but I can't see anyway that could cause the problems stayed by the OP.

 

[petersonra]

I'm not really following this. Sounds like a wye connected 690Y/400 secondary. The wye connections are all permanent and factory made inside the transformer. Even if the SBJ is missing, the neutral to phase and phase to phase voltages will all be the same and stable. Even if the system is truly floating, then of course neutral o ground could drift from 0 a bit, but I can't see anyway that could cause the problems stayed by the OP.

Agreed. I think when this gets run down it will have nothing to do with either bonding or grounding. Hopefully the OP will be decent enough to come back when he figures it out in a month or two and let us know what it was.

 

[Elect117]

The transformer's fuse can be tripping rather than the switchgear OCPD due to a couple of different reasons.

1) uncoordinated ground fault devices. The fuse setting can be lower than the OCPD at the gear
2) restriking grounds on a wye ungrounded secondary from insulation failure. This is a stretch.
3) good old fashion ground fault

If the fuse is the OCPD, it can be an issue with parallel runs, high resistance ground fault on that one phase that only gets high enough to trip the heating element of the OCPD (overload on the fuse rather than Ground fault on it), or just straight up overloaded secondary conductor or their connections.

Lastly, and the least likely, harmonics from VFDs that are causing extra heat.

Have you tried an IR gun scan on the different connection points?

 

[roger]

Even if the SBJ is missing, the neutral to phase and phase to phase voltages will all be the same and stable.

Exactly.

 

[Errorsfordays]

Is it the same "moderate load" that is blowing the fuse each time?

But I am not answering YOUR question. Is it possible for a loose connection in the pad mount or switch-gear upstream from the secondary transformer to trip that fuse? If that is what's happening, then some sparking on the primary side would be causing increased current on the load side, like an inrush to a reactive component. Since "Some of the more sensitive components on that line are also burning up intermittently." yeah, I could see that as being possible.

A thorough thermal scan might show something.

The downstream side of the 690V is a power converter consisting of a 1000+Vdc bus, IGBTs on either side of it, and a filtration circuit, connected to an asynchronous generator. The generator and cables are removed from the circuit for troubleshooting purposes. The DC bus precharges normally, and the IGBTs have all been swapped between phases to see if the fault will follow. Capacitors in the bus have been tested, and the filtration as well. The “moderate load” is a rectifier test, with inrush current being 488A(U)-356A(V)-488A(W) on 450A fuses. Only the fuse on phase U is blowing and it is intermittently between one second and five minutes.

We are going to do full inspections of the PMT and switch-gear next week, I’ll make sure to bring an IR gun, and post our findings.

 

[TwoBlocked]

Wowser! That's a 20% current unbalance! You MAY have multiple problems, but more likely just one problem with many symptoms. You swapped the IGBT's and the problem did not follow them. Maybe the boards that drive the IGBTs? Can you swap them around and see if the problem follows?

You are fortunate that the fault happens at a predictable time! Say, what brand of fuses are you using? I may want to buy stock in the company.

from: https://voltage-disturbance.com/current-unbalance-calculator/

 

[Errorsfordays]

Wowser! That's a 20% current unbalance! You MAY have multiple problems, but more likely just one problem with many symptoms. You swapped the IGBT's and the problem did not follow them. Maybe the boards that drive the IGBTs? Can you swap them around and see if the problem follows?

You are fortunate that the fault happens at a predictable time! Say, what brand of fuses are you using? I may want to buy stock in the company.

from: https://voltage-disturbance.com/current-unbalance-calculator/
View attachment 2570665

Yep, the drivers cards went with their respective IGBTs. Unfortunately some are not interchangeable due to revisions, compatibility, etc. These are SIBA fuses, and at the rate I’m going through them you may want to invest soon!

 

[TwoBlocked]

Best of luck, guy!

 

[synchro]

Are there error codes that can be read from the power converter?

 

[TwoBlocked]

... The “moderate load” is a rectifier test, ...

I'm curious: A "rectifier test" as a load? Can you explain?

The downstream side of the 690V is a power converter consisting of a 1000+Vdc bus, IGBTs on either side of it, and a filtration circuit, connected to an asynchronous generator. ...

And also curious: This power converter can be powered from either utility power, from the PMT, or from an asynchronous generator, a wind turbine? So then the DC bus provides power back through the PMT when the wind turbine is working? Or is the DC bus for other purposes?

 

[synchro]

The downstream side of the 690V is a power converter consisting of a 1000+Vdc bus, IGBTs on either side of it, and a filtration circuit, connected to an asynchronous generator. The generator and cables are removed from the circuit for troubleshooting purposes. The DC bus precharges normally, and the IGBTs have all been swapped between phases to see if the fault will follow. ... Only the fuse on phase U is blowing and it is intermittently between one second and five minutes.

... You swapped the IGBT's and the problem did not follow them. Maybe the boards that drive the IGBTs? Can you swap them around and see if the problem follows?

Yep, the drivers cards went with their respective IGBTs. Unfortunately some are not interchangeable due to revisions, compatibility, etc.

It sounds like swapping the IGBTs together with their drivers did not change which phase has the fuse that's blowing.
It might be helpful to rotate the phases that are feeding the IGBT-based rectifier (which would be like the active front end of a VFD).

If the fuse that blows moves to a different phase, then perhaps the digital control circuitry is not providing a correct signal to the driver for one of the three sets of IGBTs. For example, that signal might not have the proper timing or pulse width, or it could be intermittent.

If the fuse keeps blowing on the same phase after rotating the phases, then there might be something going on upstream of the rectifier. Just speculating, but perhaps there could be some switching spikes from the IGBTs that end up triggering a defective surge protection device on the problem phase. And then if that SPD is downstream of a fuse, the SPD might cause it to blow.

Are there any capacitors on the 690V line downstream of the fuse? If so, perhaps there could be some resonance happening that causes the fuse to blow.

 

[Errorsfordays]

This power converter can be powered from either utility power, from the PMT, or from an asynchronous generator, a wind turbine? So then the DC bus provides power back through the PMT when the wind turbine is working? Or is the DC bus for other purposes?

Correct. There are a few different types of wind turbine converters, but in this case, the stator is in direct line to the PMT and the converter is bi-directional to accommodate sub-sync and super-sync frequencies at the rotor. The rectifier test checks the switch capabilities of the IGBTs before a pole test for phase location.

 

[TX+ MASTER#4544]

electrofelon said.....even if SBJ...........

The bonding jumper should not be missing. It is vital to the grounding and bonding scheme for voltage stabilizing as well as transient voltage.

All of the grounding and bounding conductors should terminate at either the transformer or the first panelboard /switchboard but not at both transformer and PB/S Gear. This would include the grounding electrode conductor, too.

Failure to do so, will cause a parallel path for current to flow via the metallic raceway(s).

What type of grounding electrode is your grounding electrode conductor terminated on?

It is crucial that the system bonding jumper (SBJ) be properly and appropriately located at one place on the secondary side of the transformer or at the PB/Switchgear, but not both. All grounded conductors (neutrals) and grounding electrode conductor and the SBJ should terminate on the same bus bar.

Is this a three wire three phase service/feeder (no neutral) or a three phase four wire with a neutral? If it is a four wire service/feeder and a 600 "Y" volt four wire with a neutral, then the neutral voltage would be 347 volts, 600 / 1.732 = 347 volts, as per 220.5 Calculations. Thus, 600 Y/347 or 600 volts.

So, I am curious about the 400 volt and the 690 volt.

Thanks,
Comments accepted
TX+MASTER #4544

 

[Jraef]

a 12kV switch-gear, and a secondary transformer stepping down 12kV to 400V and 690V. One fuse keeps blowing on a single phase of the 690V side every time a moderate load is put on it. I

So if you have a single transformer and two voltages. Is this a 690Y400V secondary then? If so, and only ONE fuse blows, was that fuse feeding a 400V single phase load? Was the load designed for 400V L-N?